The organic light emitting diode (OLED) is a novel flat plate display device, which exhibits such performances as energy-saving, high response speed, high color stability, strong adaptability of environment, non-radiation, long lifetime, light weight, thin thickness, and so on. The research on OLED originated in the mid 1980s. For instance, the U.S. Pat. No. 4,720,432 disclosed an OLED which uses aromatic tertiary amine as the hole-transporting material and 8-Hydroxyquinoline aluminum as the light-emitting material, and which demonstrated a wide application prospect in the field of display.
Realization of a flat plate display with full-color, large size and huge information storage is one of the most vital goals of the development of OLED. Presently, the color display of OLED is mainly realized by the method of paralleling three color pixels of red, green and blue (i.e., RGB) or the method of stacking three color pixels. Although full-color OLED products based on the above methods have been on market, their fabrication processes are very complicated owing to strictly precise photoengraving and masking technologies during fabrication process, which limits the application and development of OLED.
Accordingly, more attention has been paid on the study of on full-color OLED color filter technology applying directly liquid crystal display (LCD) on white-light OLED to realize OLED full-color light emitting. This method exhibits such characteristics as simple production process and low production costs, and is in favor of commercialization of full-color OLED flat plate display. For example, the U.S. Patent Application Publication No. 2007/0115221A1 has disclosed a full-color OLED display applying the color filter technology and white-light OLED material.
The white-light OLED has a brightness of above 10,000 cd/m2, a luminous efficiency of above 40 lm/W, a turn-on voltage of less than 4 V and a lifetime of above 20,000 hours (at the brightness of 800-1,000 cd/m2). It characteristics of energy-saving, light weight and thin thickness indicate that it can be used not only as back lighting source of flat plate display, such as, liquid crystal display (LCD), but as a novel energy saving planar illuminating device to be used widely.
However, the prior art white-light OLED mainly suffers from the following problems: the luminous efficiency of the white-light OLED is still low, resulting in a low luminous efficiency of the device far lower than the practical standard of 50-80 lm/W, which does not meet the requirement for illumination; the color coordinate of white-light OLED is unstable due to the imbalance of injection of electrons and holes in it, which causes that the lighting color varies with the voltage; the lifetime of white-light OLED is still short, and is far less than the standard for lamination lifetime of 50000 hours; the production process of white-light OLED is relatively complicated, and both the material preparation process and device fabrication process need to be simplified to reduce its production costs.
In order to resolve the above problems, many device structure have been developed to meet the practical requirements of white-light OLED. For example, the U.S. Patent Application Publication No. US2006/0003184 has disclosed a white-light OLED comprising a first light-emitting layer and a second light-emitting layer. Although the device performance may be improved greatly, the fabrication process is complicated owing to great number of layers of the white-light OLED, and the device has a relative higher turn-on voltage and a relatively lower luminous efficiency. These problems may be resolved by employing a doping structure with a single light-emitting layer. However, it is difficult to precisely quantify the doping concentration of dyes. Moreover, the phase separation and interface degradation caused by the doping will shorten the lifetime of the device. Therefore, the optimal method to resolve the above problems is to prepare an ideal white-light organic electroluminescent, and to fabricate a single light-emitting layer by using it in white-light OLED. It will not only increase the luminous efficiency of the device, but can greatly simplify the fabrication process and reduce the production costs.
The white-light organic electroluminescent polymer (WOEP) is the most representative type of the prior art white-light electroluminescent materials. Such a material can be classified into three types of straight chain structure, branched chain structure and dendrite structure in its structure. For example, the Chinese Patent Application Publication No. 1852933A has disclosed a WOEP incorporating blue-light emitting repeating units and red-light emitting repeating units, or incorporating blue-light emitting repeating units, green-light emitting repeating units and red-light emitting repeating units. Among them, red-light emitting groups and green-light emitting groups can be inserted into the main chain of polyfluorene to realize the white-light emitting. Alternatively, green-light emitting pendant groups and red-light emitting pendant groups can be connected with the main chain of polyfluorene to realize the white-light emitting. In the prior art, the WOEP emits white-light by the energy transmission between light-emitting groups with a wide bandgap and light-emitting groups with a narrow bandgap. This can be utilized to fabricate a white-light OLED with a single light-emitting layer so as to resolve the problems of the prior art white-light OLEDs having a relatively multilayered structure.
However, the fluorescence quantum efficiency is relatively low (below 0.1 in general), and need to be further improved if the WOEP is used as the light-emitting layer in the OLED, which is due to the strong molecule stacking effect among adjacent chains and the fluorescence quenching effect. The white-light WOEP can readily crystallize when it is used as the light-emitting layer in the white-light OLED owing to its low glass transition temperature (below 100° C. in general) and poor thermal stability. Further, the ratio of respective light-emitting groups needs to be precisely controlled in an order of magnitude of 10−4 during synthesis of the WOEP, which greatly increases difficulty of the synthesis. Furthermore, the WOEP is difficult to be purified in a desirable purity, and it is not in favor of a mass production.